Proton Transfers in Aromatic and Antiaromatic Systems. How Aromatic or Antiaromatic Is the Transition State? An Ab Initio Study

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• 作者：Claude F. Bernasconi ; Philip J. Wenzel ; Mark L. Ragains
• 刊名：Journal of the American Chemical Society
• 出版年：2008
• 出版时间：April 9, 2008
• 年：2008
• 卷：130
• 期：14
• 页码：4934 - 4944
• 全文大小：294K
• 年卷期：v.130,no.14(April 9, 2008)
• ISSN：1520-5126

文摘

An ab initio study of six carbon-to-carbon identity proton transfers is reported. They refer tothe benzenium ion/benzene (C₆ls/jacsat/130/i14/eqn/ja078185ye10001.gif">/C₆H₆), the 2,4-cyclopentadiene/cyclopentadienyl anion (C₅H₆/C₅ls/jacsat/130/i14/eqn/ja078185ye10002.gif">),and the cyclobutenyl cation/cyclobutadiene (C₄ls/jacsat/130/i14/eqn/ja078185ye10003.gif">/C₄H₄) systems and their respective noncyclic referencesystems, that is,, and . For the aromaticC₆ls/jacsat/130/i14/eqn/ja078185ye10004.gif">/C₆H₆ and C₅H₆/C₅ls/jacsat/130/i14/eqn/ja078185ye10005.gif"> systems, geometric parameters and aromaticity indices indicate thatthe transition states are highly aromatic. The proton-transfer barriers in these systems are quite low,which is consistent with a disproportionately high degree of transition-state aromaticity. For the antiaromatic C₄ls/jacsat/130/i14/eqn/ja078185ye10006.gif">/C₄H₄ system, the geometric parameters and aromaticity indices indicate a rathersmall degree of antiaromaticity of the transition state. However, the proton-transfer barrier is higherthan expected for a transition state with a low antiaromaticity. This implies that another factor contributes to the barrier; it is suggested that this factor is angle and torsional strain in the transition state.The question whether charge delocalization at the transition state might correlate with the development of aromaticity was also examined. No such correlation was found, that is, charge delocalizationlags behind proton transfer as is commonly observed in nonaromatic systems involving -acceptorgroups.